All definitively positive COVID-19 patients with Hb≤9. Having at least one of the severe symptoms of COVID-19, including tachypnea (breathing rate> 30 beats per minute), hypoxemia (O2 ≤93 saturation, the partial pressure ratio of arterial oxygen <300), Lung infiltration (> 50% of lung field within 24 to 48 hours), progressive lymphopenia, LDH>245 U/I, CRP>100

All definitively positive COVID-19 patients with Hb≤9. Having at least one of the severe symptoms of COVID-19, including tachypnea (breathing rate> 30 beats per minute), hypoxemia (O2 ≤93 saturation, the partial pressure ratio of arterial oxygen <300), Lung infiltration (> 50% of lung field within 24 to 48 hours), progressive lymphopenia, LDH>245 U/I, CRP>100